RELATIONS BETWEEN MIX DESIGN AND FATIGUE PROPERTIES OF ASPHALTIC CONCRETE

A great deal is known about the fatigue properties of bituminous materials, but this knowledge is based on tests carried out under various conditions. Consequently, it is difficult to obtain a general picture of how the fatigue properties are influenced by the most important parameters such as binder content, void contents, grading, temperature etc. therefore, in this paper an attempt is made to generalize as far as possible and to isolate the influence of the most important parameters. The first step is to show that the influence of the three parameters, the penetration grade of the binder, the temperature and the loading time can be combined by use of one single parameter, the stiffness of the binder. Then is shown how the fatigue properties of a mix is influenced by this parameter. Next is shown that the fatigue properties improve with increasing the maximum size of the aggregate and how to isolate this factor. The next step is to show the effect of the binder content and how it is influenced by the content of voids, and a correction curve for this effect is shown. Furthermore, it is shown that a mix must contain a certain minimum of filler in order to obtain good fatigue properties. This leaves only one parameter, the shape of the gradation curve, which is of importance in so far as it determines the number of large voids in the mix, and a large number may lead to poor fatigue properties. While the fatigue properties are important in thickness design, when a bituminous base is used, they are only part of the problem in mix design, where other important points as the stiffness of the mix, which determines the load spreading capacity, the stability, the workability and the economy must be considered. This will often fix a limit to how good fatigue properties it is possible to obtain, but it is hoped that this paper will enable the mix designer to get close to that limit, which will allow the thickness designer to calculate with a high permissible strain in the material. /AUTHOR/